A massive body of evidence, from neuropathology, studies of "split- brain" patients, of "divided visual field" studies in normal subjects, and now from neuroimaging, is all consistent in showing that in right- handed individuals the primary neuronal processing of verbal material takes place in the left hemisphere, while analysis of visual form often proceeds initially in the right hemisphere This situation of "hemispheric specialization" most likely accrues from the Act that communication between the hemispheres is constrained by the very small size of most of the nerve fibers in the forebrain commissures, making it much quicker to process information intrahemispherically than to shuttle back and forth between the hemispheres. Concentrating processing in one hemisphere must also mean that the neuronal facilitation of attention is "focussed" in the relevant hemisphere and that, as is well-known for attending to visual space, disengaging this attentive focus from one locus and concentrating it on another takes a measurable amount of time. Surprisingly, this temporal cost of switching attention between the cerebral hemispheres has never been measured, and it is the intent of this proposal to do so. We will use memory for words and for nonobjective colored images to tap each hemisphere's analytic and hence attentive predilection, already having shown the high degree of similarity in the mnemonic processing of these two disparate types of material. Any increment in reaction time consequent to switching the demands of memory from one to the other type of material can be detected. This will be done using a divided visual field paradigm in which the words or images are directed initially to one or the other hemisphere, and the "switching time" can thus be disentangled from other aspects of interhemispheric communication. Knowing this switching time and its properties should greatly advance understanding of cognitive events that draw upon or demand jointly the specialized skills of the two hemispheres; and it seems likely that the ability to define this time would have wide application in evaluating a variety of cognitive disturbances, including schizophrenia and dyslexia.